A Kinetic Study of the Thiol-Disulfide Exchange Reaction between Aminothiols and 5, 5'-Dithiobis (2-nitrobenzoic acid)

Abstract

The rate constants for the thiol-disulfide exchange, k_SH, and for the thiolate iondisulfide exchange, k_s-, between biologically relevant aminothiols, i.e., cysteine, cysteamine and penicillamine, and 5, 5'-dithiobis (2-nitrobenzoic acid), Ellman's reagent, in aqueous solutions were determined by a spectrophotometric method. The apparent rate constant, k₁', increases with increase of pH under the experimental conditions used. The value of k_s-, approximately 2×10⁵ M^-1S^-1 except for penicillamine, appears to be independent of the pK_a (SH), and k_SH is three orders of magnitude smaller than k_s-. The thiolate ion-disulfide exchange in penicillamine is one order of magnitude slower than in the other aminothiols. The results of the kinetic study indicate that one of the sulfur atoms of disulfide is attacked by the thiolate anion with subsequent cleavage of the S-S bond.